1. Technical Field
The present disclosure generally relates to well bore tools and in particular to apparatus and methods for collecting downhole samples.
2. Background Information
Oil and gas wells have been drilled at depths ranging from a few thousand feet to as deep as 5 miles. Wireline and drilling tools often incorporate various sensors, instruments and control devices in order to carry out any number of downhole operations. These operations may include formation testing and monitoring and tool monitoring and control.
Formation testing tools have been used for monitoring formation pressures along well boreholes, obtaining formation fluid samples, and predicting performance of reservoirs. Such formation testing tools typically contain an elongated body having an elastomeric packer and/or pad that is sealingly pressed against a zone of interest in the borehole to collect formation fluid samples in fluid receiving chambers placed in the tool. The borehole can be sealed off, either completely or partially, from the formation with a mud cake formed by the drilling fluid. The formation testing tool can be sealingly pressed against the borehole wall with the mud cake providing a seal between the formation testing tool and the borehole wall.
Formation testing tools have been developed with extendable sampling probes for engaging the borehole wall at the formation of interest for withdrawing fluid samples from the formation and for measuring pressure. In formation testing tools of this nature an internal pump or piston may be used after engaging the borehole wall to reduce pressure at the formation tool interface causing fluid to flow from the formation into the formation tool. The drilling fluid is circulated through an inner bore of a drill string and returns to the surface through an annular space between the drill string and an inner wall of the borehole. The return fluid includes solids and liquids. The high pressure of the return fluid column forces liquids into the formation and the solids tend to accumulate along the borehole wall forming the mud cake that can reduce or prevent contaminants present in the borehole wall from leaking off into the formation. In many cases, however, it is still possible for the contaminants to leak through the mud cake and into the formation even after the mud cake is formed, thereby contaminating the formation and formation samples. In some cases the mud cake is damaged and in some cases the mobility or permeability of the mud cake remains too high to adequately prevent invasion of the formation. In addition, the seal between the mud cake and the elastomeric packer and/or pad/probe can be poor, which can lead to drilling fluid leaking into the formation and/or the downhole sample as it is acquired. There is a need, therefore, for improved apparatus and methods for reducing the potential for drilling fluid and other impurities from contaminating downhole samples.